Consistent diel patterns in foraging depths have been recorded in several avian predators which feed on pelagic prey, but nothing is known about such effects in bottom feeding seabirds. In the underwater environment, illumination is influenced primarily by water depth. Thus a predator's ability to exploit feeding areas located in deep water could, theoretically, be constrained by light availability. We investigated the effects of the diel light cycle on the diving behaviour of 2 species of bottom feeding seabird: the blue-eyed shag Phalacrocorax atriceps and the European shag P. aristotelis. Time-based data loggers were attached to 22 blue-eyed shags and 21 European shags and used to obtain detailed and concurrent records of foraging conditions in terms of depth and illumination during the chick-rearing period. Both species were exclusively diurnal feeders. There were marked differences in mean foraging depth and foraging illumination between individual birds (blue-eyed shag: ranges 3.2 to 73.3 m and -0.3 to 2.0 log10 lx; European shag: 8.5 to 34.6 m and 0.9 to 2.0 log10 lx). Superimposed on this variation was a consistent effect whereby individuals significantly modified their diving behaviour in response to the diel light cycle, in accord with the prediction that foraging depths would be maximised when ambient illumination was highest. However, it appeared that individuals did not fully exploit the potential of this diel effect since, although foraging depths in the middle of the day were generally deeper, the associated illumination was higher than that experienced during shallower dives made earlier or later in the day. Nevertheless, we believe that diel effects can play an important role in shaping the foraging behaviour of bottom feeding, avian predators.